ABSTRACT Natural Killer (NK) cells are cytotoxic lymphocytes that mediate immune defense against viral pathogens and tumors. In clincial settings, NK cells are being targeted in adoptive cell transfer (ACT) and hematopoietic stem cell transplantation (HSCT) regimens, and immunotherapeutic approaches, to treat cancer and cancer-related infections. Effective NK cell immunity is predicated on the sustained presence of a pool of functional NK cells in the periphery. At steady state, the NK cell niche is maintained by the continual development of new NK cells from progenitor populations in the bone marrow and by the homeostatic expansion of mature NK cells in the periphery. In addition, certain inflammatory signals may transiently expand effector NK cells needed for protection during infection or malignancy. Understanding the molecular pathways that control these processes will critically impact the development of novel and more effective NK cell-based therapies in the clinic. Our preliminary studies have highlighted a critical role for the Inhibitor of Apoptosis Protein (IAP), Birc5, in NK cell hematopoiesis and function. Prior studies in stem cells and cancer cells have shown that Birc5 contributes to a broad range of cellular functions, including mitosis, survival, and cellular metabolism, acting through its highly compartmentalized activities in the nucleus, cytoplasm, and mitochondria, respectively. A role for Birc5 in NK cell biology has not been described. New data in our lab indicate that Birc5 is highly and transiently upregulated in NK cells undergoing expansion in the context of development in the bone marrow, or viral infection in the periphery. Further, our preliminary studies indicate that Birc5-deficiency severely impairs mouse NK cell development and maturation. The specific cellular functions of Birc5 in NK cells, as well as its roles in homeostatic- and infection-related NK cell expansion remain to be defined. Thus, we will conduct studies that test the central hypothesis that Birc5 acts downstream of key cytokine and metabolic growth signals to orchestrate NK cell expansion during development and peripheral immunity. These studies will utilize novel transgenic mice, in which the Birc5 gene can be deleted in NK cells in a cell-specific or conditional manner, to investigate the contributions of Birc5 to NK cell proliferation, survival, and metabolic function in settings of development, homeostatic expansion, and anti-viral immune responses in vivo. Further, we will combine sophisticated molecular and genetic techniques to define the environmental cues and intracellular signaling networks that modulate Birc5 function in NK cells. And finally, we will use an innovative mouse xenograft model to study Birc5 function in human NK cells in vivo. Ultimately, our studies have the potential to inform ongoing and future NK cell-based immunotherapies, including adoptive cell transfer (ACT) and hematopoietic stem cell transfer (HSCT) regimens that harness the anti-tumor properties of NK cells to treat cancer. !